Control of directionality in atomic layer etching

ABSTRACT

A method for performing atomic layer etching (ALE) on a substrate is provided, including the following operations: performing a surface modification operation on a substrate surface, the surface modification operation configured to convert at least one monolayer of the substrate surface to a modified layer, wherein a bias voltage is applied during the surface modification operation, the bias voltage configured to control a depth of the substrate surface that is converted by the surface modification operation; performing a removal operation on the substrate surface, the removal operation configured to remove at least a portion of the modified layer from the substrate surface, wherein removing the portion of the modified layer includes applying thermal energy to effect desorption of the portion of the modified layer. A plasma treatment can be performed to remove residues from the substrate surface following the removal operation.

CLAIM OF PRIORITY

This application claims priority as a continuation of U.S. applicationSer. No. 15/615,691, filed Jun. 6, 2017, entitled “CONTROL OFDIRECTIONALITY IN ATOMIC LAYER ETCHING.” U.S. application Ser. No.15/615,691 claims priority to U.S. application Ser. No. 15/423,486,filed Feb. 2, 2017, entitled “ATOMIC LAYER ETCHING 3D STRUCTURES: SI ANDSIGE AND GE SMOOTHNESS ON HORIZONTAL AND VERTICAL SURFACES,” whichclaims priority to U.S. Provisional Application No. 62/291,392, filedFeb. 4, 2016, entitled “ATOMIC LAYER ETCHING 3D STRUCTURES: SI AND SIGEAND GE SMOOTHNESS ON HORIZONTAL AND VERTICAL SURFACES.” U.S. applicationSer. No. 15/615,691 further claims priority to U.S. ProvisionalApplication No. 62/464,360, filed Feb. 27, 2017, entitled “CONTROL OFDIRECTIONALITY IN ATOMIC LAYER ETCHING.” The disclosures of theseapplications are incorporated by reference herein.

FIELD OF THE INVENTION

Implementations of the present disclosure relate to atomic layer etching(ALE), and more specifically to control of directionality in atomiclayer etching.

DESCRIPTION OF THE RELATED ART

Conventional techniques of etching material on semiconductor substrateswith fine-tuned control over the uniformity and etch rate are limited.For example, reactive ion etch is conventionally used to etch materialson a semiconductor substrate during semiconductor processing and etchrates of materials etched using reactive ion etch are controlled bymodulating radio frequency plasma power and chemistry selection.Typically, a wafer plasma sheath forms at the top of the substrate, andthus ions from the plasma are typically accelerated onto the wafersurface to etch the substrate. However, as technology nodes progress toatomic-scale devices, control of etch processes with atomic-scalefidelity will be required.

SUMMARY

In accordance with some implementations, a method for performing atomiclayer etching (ALE) on a substrate is provided, including the followingoperations: performing a surface modification operation on a substratesurface, the surface modification operation configured to convert atleast one monolayer of the substrate surface to a modified layer,wherein a bias voltage is applied during the surface modificationoperation, the bias voltage configured to control a depth of thesubstrate surface that is converted by the surface modificationoperation; performing a removal operation on the substrate surface, theremoval operation configured to remove at least a portion of themodified layer from the substrate surface, wherein removing the portionof the modified layer is effected via a ligand exchange reaction that isconfigured to volatilize the portion of the modified layer.

In some implementations, the surface modification operation isconfigured to diffuse ions into the substrate surface to the depth ascontrolled by the bias voltage.

In some implementations, the bias voltage is configured to have amagnitude and a time duration during the surface modification operationto achieve the depth of the substrate surface that is converted by thesurface modification operation.

In some implementations, the depth is defined by one or more monolayersof the substrate.

In some implementations, the bias voltage is configured to shift thesurface modification operation from being primarily isotropic to beingprimarily anisotropic, depending on a magnitude of the bias voltage.

In some implementations, the bias voltage is applied during part of thesurface modification operation, the part during which the bias voltageis applied to increase an amount of the depth in a vertical directionthat increases anisotropy of the ALE, and a portion during which thebias voltage is not applied to increase the depth in a non-verticaldirection that increases isotropy of the ALE.

In some implementations, the method further includes: performing,following the removal operation, a plasma treatment on the substratesurface, the plasma treatment configured to remove residues generated bythe removal operation and/or the surface modification operation from thesubstrate surface, wherein the residues are volatilized by the plasmatreatment.

In some implementations, the removal operation is configured to removeless than an entire portion of the modified layer from the substratesurface; and, the method further comprising: repeating the removaloperation and the plasma treatment until the entire portion of themodified layer is removed from the substrate surface.

In some implementations, the method further includes: repeating thesurface modification operation, the removal operation and the plasmatreatment until a predefined thickness has been etched from thesubstrate surface.

In some implementations, the bias voltage is in the range ofapproximately 20 to 100 V.

In some implementations, performing the surface modification operationincludes exposing the substrate surface to a fluorine-containing plasma,wherein the exposure to the fluorine-containing plasma is configured toconvert the at least one monolayer of the substrate surface to afluoride species.

In some implementations, the substrate surface includes a metal, metaloxide, metal nitride, metal phosphide, metal sulfide, metal arsenide, ormetal compound; wherein the exposure to the fluorine-containing plasmaforms a metal fluoride.

In some implementations, exposing the substrate surface to thefluorine-containing plasma includes introducing a fluorine-containinggas into a chamber in which the substrate is disposed, and igniting aplasma.

In some implementations, the exposure to the fluorine-containing plasmais performed at a chamber pressure of about 10 to 500 mTorr, for aduration of less than about 15 seconds.

In some implementations, performing the removal operation includesexposing the substrate surface to tin-(II) acetylacetonate (Sn(acac)₂)vapor, the exposure to the Sn(acac)₂ vapor configured to exchangeacetylacetonate (acac) ligands for fluorine atoms in the modified layer.

In some implementations, exposing the substrate surface to the Sn(acac)₂includes introducing the Sn(acac)₂ as a vapor into a chamber in whichthe substrate is disposed.

In some implementations, the exposure to the Sn(acac)₂ is performed fora duration of about 1 to 30 seconds.

In some implementations, performing the plasma treatment includesexposing the substrate surface to a hydrogen plasma, the exposure to thehydrogen plasma configured to volatilize tin, tin fluoride or tin oxideresidues on the substrate surface.

In some implementations, exposing the substrate surface to the hydrogenplasma includes introducing a hydrogen gas into a chamber in which thesubstrate is disposed, and igniting a plasma.

In some implementations, the exposure to the hydrogen plasma isperformed for a duration of about 1 to 30 seconds.

In some implementations, the surface modification operation is performedin a first chamber; wherein the removal operation is performed in asecond chamber.

In some implementations, a method for performing atomic layer etching(ALE) on a substrate is provided, including the following operations:performing a surface modification operation on a substrate surface, thesurface modification operation including exposing the substrate surfaceto a first plasma that converts at least one monolayer of the substratesurface to a modified layer, wherein a bias voltage is applied duringthe surface modification operation, the bias voltage being configured tocontrol a depth of the substrate surface that is converted by thesurface modification operation, wherein the bias voltage is configuredto accelerate ions from the first plasma towards the substrate surfacewithout substantially etching the substrate surface; performing aremoval operation on the substrate surface, the removal operationincluding removing at least a portion of the modified layer from thesubstrate surface, wherein removing the portion of the modified layer iseffected via a ligand exchange reaction that is configured to volatilizethe portion of the modified layer; performing a clean operation on thesubstrate surface, the clean operation including removing residuesgenerated by the removal operation from the substrate surface, the cleanoperation further including exposing the substrate surface to a secondplasma, wherein the residues are volatilized by the exposure to thesecond plasma.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates various classifications of ALE, in accordance withimplementations of the disclosure.

FIG. 2 illustrates the chemical reaction of an ALE process for etchingAl₂O₃, in accordance with implementations of the disclosure.

FIG. 3 illustrates an ALE process for etching Al₂O₃, in accordance withimplementations of the disclosure.

FIGS. 4A-4C illustrate performance of ALE process operations in a plasmatreatment chamber and a vapor treatment chamber, in accordance withimplementations of the disclosure.

FIG. 5 illustrates a process flow diagram for a method performed inaccordance with disclosed implementations.

FIG. 6 illustrates a graph showing Al Oxide thickness and Aloxy-fluoride thickness under various conditions, using ARXPScharacterization of the surface following single fluorination, inaccordance with implementations of the disclosure.

FIG. 7 illustrates surface fluorination depth using zero-bias plasma, inaccordance with implementations of the disclosure.

FIGS. 8A-E are STEM images of a cross section of a fluorinated film, inaccordance with implementations of the disclosure.

FIGS. 9A and 9B illustrate film loss following a 30 second fluorineplasma exposure versus a 300 second fluorine plasma exposure,respectively, in accordance with implementations of the disclosure.

FIG. 10 is a graph showing SE characterization of film loss,demonstrating the self-limiting nature of the fluorination and Sn(acac)2exposure, in accordance with implementations of the disclosure.

FIG. 11 is a graph illustrating calculated fluorination depth as afunction of fluorine ion energy, in accordance with implementations ofthe disclosure.

FIG. 12 illustrates a method for performing ALE using multiple ligandexchange and plasma cleaning operations per single surface modificationoperation, in accordance with implementations of the disclosure.

FIG. 13A conceptually illustrates a cross section of a substrate surfacefeature, and performance of an anisotropic ALE process performedthereon, in accordance with implementations of the disclosure.

FIG. 13B conceptually illustrates a cross section of a substrate surfacefeature, and performance of an isotropic ALE process performed thereon,in accordance with implementations of the disclosure.

FIGS. 14A-D illustrate a process for providing increased anisotropythrough deposition of a passivation layer, in accordance withimplementations of the disclosure.

FIG. 15 illustrates a cluster tool 1500, in accordance withimplementations of the disclosure.

FIG. 16 illustrates an example etching chamber or apparatus, inaccordance with implementations of the disclosure.

FIG. 17 shows a control module for controlling the systems describedabove, in accordance with implementations of the disclosure.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a thorough understanding of the presented implementations. Thedisclosed implementations may be practiced without some or all of thesespecific details. In other instances, well-known process operations havenot been described in detail to not unnecessarily obscure the disclosedimplementations. While the disclosed implementations will be describedin conjunction with the specific implementations, it will be understoodthat it is not intended to limit the disclosed implementations.

Provided herein are methods of controlling directionality of atomiclayer etching (ALE) of metal oxides (such as aluminum oxide (Al₂O₃)) viaa ligand exchange mechanism involving a fluorine-containing plasma and atin-containing etchant. Methods described herein involve modifying asurface of the material to be etched using a fluorine-containing plasmaand exposing the modified surface to tin-(II) acetylacetonate(Sn(acac)₂) vapor to remove the material in a self-limiting manner. Aligand exchange reaction is sustained in a vapor deposition chamber withSn(acac)₂ vapor without plasma.

Atomic layer etching (ALE) is one approach for atomic scale control ofetching behavior. ALE is a type of cycling process. ALE is a techniquethat removes thin layers of material using sequential self-limitingreactions. Generally, ALE may be performed using any suitable technique.Examples of atomic layer etch techniques are described in U.S. Pat. No.8,883,028, issued on Nov. 11, 2014; and U.S. Pat. No. 8,808,561, issuedon Aug. 19, 2014, which are herein incorporated by reference forpurposes of describing example atomic layer etch and etching techniques.In various implementations, ALE may be performed with plasma, or may beperformed thermally.

An ALE process sequence can be described as follows, in accordance withimplementations of the disclosure. Initially, a portion of a surface ofa substrate is in an unmodified state. The outermost layer (or surfacelayer) of molecules/atoms of the substrate surface are exposed for theALE process. A surface conversion/modification operation is performed toconvert the surface layer of the substrate to a functionalized state.For example, the surface layer is modified by exposure to a surfaceconversion reactant, which may adsorb or chemisorb on the surface. Thesurface conversion reactant can include molecules or low energy radicalsin various implementations, which react with the surface layer atoms toeffect the surface conversion operation. The resulting surface layerincludes a functionalized outermost layer of molecules to enablesubsequent ALE steps. In some implementations, the operation isself-limiting, and only (or substantially only) the outermost layer ofthe substrate surface will undergo conversion. In some implementations,the specific depth of the conversion is controlled, at least in part viaapplication of a bias voltage which also affects directionality of theconversion, as described in further detail below. In someimplementations, this surface conversion entails conversion of thesurface species to a halide. In some implementations, following the(self-limiting) surface conversion, the chamber is purged to remove anyreaction byproducts or excess surface conversion reactant.

Following the surface conversion operation, then a ligand exchangereaction/operation is performed. The modified surface layer of thesubstrate is exposed to a ligand containing reactant, which effects aligand exchange reaction wherein the ligand containing reactant adsorbson the substrate surface and transfers its ligands to the convertedsurface species which were formed during the earlier surfaceconversion/modification operation. The ligands bond with the modifiedsurface layer of molecules/atoms, forming a reaction product includingligand substituted surface species, which can be released.

Desorption drives removal of the outermost layer of surface species (thereaction product following the ligand exchange operation) from thesubstrate surface. In some implementations, the release can be achievedby the application of thermal energy, which can be appliedsimultaneously with the exposure to the ligand containing reactant or ina separate step (e.g. by heating the chuck/chamber, lamp heating, etc.).

The concept of an “ALE cycle” is relevant to the discussion of variousimplementations herein. Generally, an ALE cycle is the minimum set ofoperations used to perform an etch process one time, such as etching amonolayer or a predefined thickness of the outer layer of the substrate.The result of one cycle is that at least some of a film layer on asubstrate surface is etched. Typically, an ALE cycle includes amodification operation to form a reactive layer, followed by a removaloperation to remove or etch, in whole or in part, only this reactivelayer.

Modification may be performed by using a chemisorption mechanism,deposition mechanism, top layer conversion mechanism, or extractionmechanism. The cycle may include certain ancillary operations such assweeping one of the reactants or byproducts. Generally, a cycle containsone instance of a unique sequence of operations.

As an example, a method for an ALE cycle may include the followingoperations: (i) delivery of a reactant gas, (ii) optional purging of thereactant gas from the chamber, (iii) delivery of a removal gas and anoptional plasma, and (iv) optional purging of the chamber. Furtherdescription and examples of ALE are described in U.S. patent applicationSer. No. 14/696,254, filed on Apr. 24, 2015 and titled “INTEGRATINGATOMIC SCALE PROCESSES: ALD (ATOMIC LAYER DEPOSITION) AND ALE (ATOMICLAYER ETCH),” which is incorporated herein by reference for purposes ofdescribing atomic layer etch processes.

Disclosed implementations result in highly controlled etching methodswith a high degree of uniformity. Disclosed implementations may be usedto perform isotropic etching of various materials and may also bemodified to perform anisotropic etching by applying a bias at a biasvoltage between about 20 V_(b) and about 80 V_(b), such as at about 50V_(b).

ALE may be done by a surface modification operation (e.g., chemisorptionby reactive chemistry on a substrate surface) followed by a removaloperation. Such operations may be repeated for a certain number ofcycles. During ALE, the reactive chemistry and the removal chemistry aredelivered separately to the substrate.

Isotropic atomic layer etching (ALE) of Al₂O₃ has been demonstrated viaa ligand exchange method utilizing a fluorine plasma for the surfacemodification step and tin-(II) acetylacetonate (Sn(acac)₂) vapor for thenon-plasma removal step. In implementations wherein the steps areperformed in the absence of any directional energy to the wafer, such asprovided via ion bias, the overall etch process is isotropic. However,in accordance with implementations of the present disclosure, anisotropycan be introduced in a controlled way to an isotropic baseline processthrough the controlled application of a bias voltage.

In various implementations, processes are performed in suitable processequipment/chambers (e.g. Kiyo for fluorination, and ICS for vaportreatment, both of which are manufactured by Lam Research Corporation).

Atomic layer etching of Al₂O₃ using sequential plasma fluorination andself-limiting thermal reactions with tin(II)-acetylacetonate (Sn(acac)₂)has been demonstrated. One approach for performing ALE of Al₂O₃ is toperform a spatial ALE process wherein the wafer (with AL₂O₃ top layer)is cycled between a plasma treatment chamber (for performingfluorination) and a vapor treatment chamber (for performing removal ofAlF₃ with Sn(acac)₂ vapor) without breaking vacuum. Another approach forperforming ALE is to perform both the plasma treatment and the vaportreatment in a single chamber, so that the wafer does not need to bemoved between different chambers.

FIG. 1 illustrates various classifications of ALE, in accordance withimplementations of the disclosure. Broadly speaking, in a generic ALEprocess, a modification operation is performed, followed by a removaloperation. The purpose of the modification operation is to weaken thesurface layer without actually etching it. One technique for modifyingthe surface is via chemisorption, which is self-limiting by Langmuirkinetics. Another way to modify the surface for ALE is via deposition.In this case, the deposition is not necessarily self-limited unless itis ALD. Even so, the removal step can be limited by reactantavailability. A third way for performing the surface modification is viaa conversion reaction. One example of a conversion is halogenation ofthe top layer. This is a diffusion-limited process and can be performedvia a plasma, bath, or other methods.

Depending on the specifics in the removal step, e.g. ion assisted or vialigand exchange, it is possible to obtain directional or isotropic ALE.

One use case for ALE is in addressing a problem known as the “four-colorchallenge.” Broadly speaking, the four-color challenge poses the problemof removing one specific color out of four without corner rounding,wherein each color represents a different material.

By way of example, isotropic ALE could enable etching of a single“color” via a ligand exchange involving transmetalation.

An unstable reaction by-product or a non-existent ligand exchangemechanism would prevent etching of the other three “colors” therebyproviding selectivity to the etched color.

Table I below provides examples of ligand exchange pre-cursors,including Sn(acac)₂, Al(CH₃)₃, AlCl(CH₃)₂, SiCl₄, and the amount ofmaterial removed per cycle, as demonstrated with reference to Y. Lee, C.Huffman, S. M. George, “Selectivity in Thermal Atomic Layer EtchingUsing Sequential, Self-Limiting Fluorination and Ligand-ExchangeReactions”, Chem. Mater., 2016, 28 (21), pp 7657-7665. As shown, etchselectivity is also achievable depending upon the particular pre-cursorutilized.

TABLE I Modification Removal Molecule Molecule Etching No Etching HFSn(acac)₂ Al₂O₃ (0.23 Å) SiO₂ ZrO₂ (0.14 Å) SiN HfO₂ (0.06 Å) TiNAl(CH₃)₃ Al₂O₃ (0.45 Å) SiO₂ (TMA) HfO₂ (0.10 Å) SiN TiN ZrO₂ Al(CH₃)₂ClZrO₂ (0.96 Å) SiO₂ (DMAC) HfO₂ (0.77 Å) SiN Al₂O₃ (0.32 Å) TiN SiCl₄ZrO₂ (0.14 Å) SiO₂ HfO₂ (0.05 Å) SiN TiN Al₂O₃

FIG. 2 illustrates the chemical reaction of a removal step of an ALEprocess for etching Al₂O₃, in accordance with implementations of thedisclosure. As has been noted, the surface portion of Al₂O₃ is firstconverted to AlF₃ by performing a surface modification/conversion step.Then, as shown, tin(II)-acetylacetonate (vapor) is provided to reactwith the aluminum(III)-fluoride (solid), to yieldtin(II)-fluoro-acetylacetonate and aluminum(III)-acetylacetonate, bothof which are volatile at the chosen process temperature. As the reactionproducts are volatile, they can be removed from the surface andevacuated from the chamber.

FIG. 3 illustrates an ALE process for etching Al₂O₃, in accordance withimplementations of the disclosure. Initially, a substrate with an Al₂O₃surface is situated in a plasma treatment chamber. Then the Al₂O₃surface is fluorinated, by way of example, utilizing a 0-bias ICP plasma(i.e. no RF power applied to the wafer pedestal). After completion, thesubstrate is moved, without breaking vacuum, into a vapor treatmentchamber in which the Sn(acac)₂-based ligand exchange reaction with thefluorinated surface takes place. A final chamber pump-out step completesthe first ALE cycle after which the wafer can be shuttled back to theplasma treatment chamber for the next ALE cycle. Alternatively, allcycles can be performed in a single chamber.

FIGS. 4A-4C illustrate performance of ALE process operations in a plasmatreatment chamber 400 and a vapor treatment chamber 410, in accordancewith implementations of the disclosure. With reference to FIG. 4A, thesubstrate 402 is shown atop a substrate holder 404 in a plasma treatmentchamber 400. Following initiation of process gas flow through a feed gasshower head 408 and warm-up, an inductively coupled plasma (ICP) isgenerated by applying power to the ICP coil 406. In variousimplementations, fluorine plasma type 1 or 2 or other types can be usedfor fluorination. It will be appreciated that a fluorine plasma can begenerated from various flurorine-containing precursors, such as CF₄,NF₃, SF₆, CHF₃, C₂H₂F₄, F₂, SiF₄, etc. In some implementations, thesubstrate holder 404 is heated to a temperature of about 100° C. In someimplementations, the fluorination operation is performed at a pressureof approximately 20 mTorr. Following the plasma exposure, a pump-out isperformed to remove process gases from the chamber.

With reference to FIG. 4B, after completion of the fluorinationoperations in the plasma treatment chamber 400, the substrate 402 ismoved, without breaking vacuum, to a vapor treatment chamber 410 forperformance of a ligand exchange operation. Following a warm-up, a vaportreatment is applied by flowing a vapor over the substrate as it isdisposed atop the substrate holder 412. For example, Sn(acac)2 vapor canbe generated by a vaporizer 414 flowed through a heated vapor line 416and distributed over the substrate via a vapor nozzle plate 418. Thevapor treatment does not entail generation of a plasma. In someimplementations, Sn(acac)2 vapor for ligand exchange is applied. In someimplementations, the substrate holder 412 is heated to a temperature ofapproximately 200° C. In some implementations, the chamber pressure ismaintained at approximately 20 mTorr to 120 mTorr. In someimplementations, the vapor treatment is applied for approximately onesecond to approximately 15 seconds. Following application of the vaportreatment, a pump-out is performed to remove process gases from thevapor treatment chamber 410.

With reference to FIG. 4C, after completion of the ligand exchangeoperation, the substrate 402 is moved, without breaking vacuum, to aplasma treatment chamber, which may be the same plasma treatment chamber400 as that utilized for the fluorination operation, or a differentplasma treatment chamber. A hydrogen plasma treatment is performed inorder to remove residual tin from the substrate surface. H₂ gas isflowed through the feed gas shower head 408 and power is applied to theICP coil 406 to generate the H₂ plasma. In some implementations, 500 WICP power is applied. In some implementations, the substrate holder 404is heated to a temperature of approximately 100° C. In someimplementations, the hydrogen plasma treatment is performed at a chamberpressure of approximately 20 mTorr. In some implementations, thehydrogen plasma treatment is performed for a duration of approximately 5to 45 seconds. Following the plasma exposure, a pump-out is performed toremove process gases from the plasma treatment chamber.

Though in the illustrated implementation, separate chambers for plasmatreatment and vapor treatment have been shown, it will be appreciated bythose skilled in the art that in other implementations, a single chambercan be used for plasma and vapor treatments. Such a system can haveappropriate valves to enable switching between different process gases(e.g. individual valves controlling the introduction of each process gasinto the chamber). Purge or pump-out operations can be performedfollowing each of the fluorination, vapor treatment, and hydrogen plasmatreatments.

A process flow diagram for a method performed in accordance withdisclosed implementations is provided in FIG. 5. During operations501-507, an inert gas such as an argon gas may be continuously flowed inthe background as a carrier gas.

In operation 501, a substrate including a material to be etched isexposed to a fluorine-containing plasma to modify the surface of thesubstrate.

The fluorine-containing plasma may be generated by introducing afluorine-containing gas and igniting a plasma. For example, in someimplementations, the fluorine-containing gas may be carbon tetrafluoride(CF₄), nitrogen trifluoride (NF₃), sulfur hexafluoride (SF₆), fluorine(F₂), or any fluorine-containing gas. In various implementations, CF₄may be introduced with O₂ to generate an abundance of fluorine ions inthe plasma to etch the substrate. In some implementations, about 35% ofthe total flow of gases to the chamber to generate thefluorine-containing plasma is O₂ gas. Other fluorine-containing gasesthat include carbon may be used in some implementations when introducedwith another gas to inhibit the formation of a carbide. For example,other fluorine-containing gases may have the formula C_(x)H_(y)F_(z),where x may be any integer greater than or equal to 1, y may be anyinteger greater than or equal to 0, and z may be any integer greaterthan or equal to 1. Examples include fluoroform (CHF₃) anddifluoromethane (CH₂F₂). In some implementations, thefluorine-containing gas may be generated by vaporizing afluorine-containing liquid.

In some implementations, the substrate is not patterned. In otherimplementations, the substrate may be patterned. The substrate mayinclude a transistor structure which may include an additional gatelayer such as a blocking oxide or an etch stop layer. For example, thesubstrate may include an aluminum oxide layer over a fin of a FinFETtransistor. In some implementations, the substrate may include a 3D NANDstructure with a metal oxide etch stop layer at the bottom of trenchesformed in the structure such that the metal oxide etch stop layer is thematerial to be etched. In various implementations, features on thesubstrate may have an aspect ratio between about 1.5:1 and about 5:1. Insome implementations, features may have aspect ratios up to about 10:1.

The plasma in operation 501 may be generated in situ or may be a remoteplasma. In many implementations, the plasma is generated in situ togenerate an inductively coupled plasma.

However, in other implementations, a capacitively coupled plasma (CCP)can be employed. In such implementations, the CCP reactor can beconfigured to enable a low-bias mode, to provide for isotropic ALE. Forexample, such a CCP reactor may employ an RF electrode on top of thereactor, a substrate holder configured to have a floating ground, andrun at a relatively high RF frequency setting, e.g. 60 MHz.

In various implementations, the substrate includes a metal oxide, metalnitride, metal phosphide, metal sulfide, metal arsenide, pure metal orany other metal compound layer to be etched. Examples include aluminumoxide (Al₂O₃) and hafnium oxide. Note that in many implementations,silicon-containing material (e.g., silicon oxide, silicon nitride,silicon carbide, silicon, etc.) may not be etched using disclosedimplementations, which contributes to achieving etch selectivityparticularly when etching a material such as a sacrificial gate oxidelayer over a fin on a FinFET transistor structure. Although it will beunderstood that disclosed implementations may be used to etch variousmaterials, FIG. 5 will be described with respect to etching aluminumoxide.

In some implementations, operation 501 may be performed without applyinga RF bias to allow isotropic modification of the substrate surface. Notethat although some disclosed implementations may be used to performisotropic etch, in other implementations, an anisotropic etching processmay also be performed by applying a bias during operation 501. Themethod described herein with respect to FIG. 5 can thus be configuredfor isotropically or anisotropically etching aluminum oxide.

Without being bound by a particular theory, during operation 501, ametal oxide surface such as an aluminum oxide surface, may befluorinated by the fluorine-containing plasma, isotropically oranisotropically, to modify the surface of the aluminum oxide to formaluminum fluoride (e.g., AlF₃). One or a few monolayers of the aluminumoxide surface may be modified to form aluminum fluoride. Themodification operation may be limited by the depth ofdiffusion/penetration of fluorine ions. Under the influence of a bias,the penetration/diffusion depth of the fluorine ions becomes deeper(along the bias axis) and also more anisotropic. The substrate may beexposed to the fluorine-containing plasma at a chamber pressure betweenabout 10 mTorr and about 100 mTorr, such as at about 20 mTorr for aduration less than about 15 seconds but greater than 0 seconds.

In another implementation, a thermal fluorination operation isperformed, as opposed to the plasma-driven process described above. Thatis, the substrate is exposed to a fluorine-containing gas (e.g. NF₃,etc.) at a sufficient temperature to induce surface fluorination,without the need for generating a plasma.

Note that in some implementations, after performing operation 501, thechamber housing the substrate may not be purged. In someimplementations, the substrate may be purged.

In operation 503, the substrate is exposed to tin-(II) acetylacetonate(Sn(acac)₂) vapor. In various implementations, Sn(acac)₂ may bevaporized in an external vaporizer prior to delivering the vapor to thesubstrate.

Without being bound by a particular theory, it is believed that when themodified AlF₃ surface is exposed to Sn(acac)₂ vapor, a ligand exchangereaction occurs such that one acac ligand on Sn(acac)₂ replaces onefluorine atom on a AlF₃ molecule, forming AlF₂(acac). AdditionalSn(acac)₂ and/or Sn(acac) may then react with AlF₂(acac) again twice toreplace the second and third fluorine atoms with (acac), resulting inAl(acac)₃. It is believed that as the acac ligands are substituted forthe fluorine atoms, the Al(acac)_(x) species becomes increasinglyvolatile, enabling it to be etched from the substrate. The reaction isself-limiting, and it is believed that some tin, tin fluoride, tinoxide, and Sn(acac)₂ may begin to build up on the surface of thematerial to be etched, thus blocking further etching of any modifiedunderlayers of AlF₃.

In some implementations, operations 501 and 503 may be performed in thesame chamber. In such implementations, a rapid temperature changebetween the ligand exchange step and the H₂ plasma flash is achieved, asthe ligand exchange reaction needs to be above approximately 190 C,whereas the H₂ plasma flash must be below approximately 150 C or elseetching of the Al₂O₃ with the H₂ plasma will occur. In operation 503,the plasma is turned off and the fluorine-containing gas flow may beturned off prior to turning on the vapor flow. In some implementations,the chamber is not purged prior to operation 503.

In some implementations, operations 501 and 503 may be performed inseparate chambers of the same apparatus. An apparatus having multiplechambers for performing ALE operations can be provided, in accordancewith implementations of the disclosure. In various implementations, thesubstrate may be shuttled or moved between a first chamber for exposingto a fluorine-containing plasma in operation 501 to a second chamber forexposing to Sn(acac)₂ vapor in operation 503. In some implementations,the second chamber is a vapor deposition chamber. In someimplementations, the second chamber is a modified chamber that does notinclude a plasma source. Note that movement or shuttling of thesubstrate between chambers may be performed without breaking vacuum.

In alternative implementations, the substrate may be exposed to anotherchemical in vapor phase that is selective to the metal fluoride but doesnot react with the metal oxide. The chemical may include one or moreligands that, when reacted with a metal fluoride, generates a volatilecompound including the metal bonded to the ligand (e.g. Sn(acac)₂).

In some implementations, operation 503 may be performed for a durationof about 1 second with the temperature of the wafer holder or pedestalholding the wafer set to a temperature of about 200° C. In variousimplementations, the chamber pressure at the end of the exposure to theSn(acac)₂ vapor may be about 20 mTorr.

In operation 505, the substrate may be exposed to a plasma treatment(e.g. a hydrogen plasma). Without being bound by a particular theory, itis believed that operation 505 is performed to volatilize tin, tinfluoride or tin oxide buildup on the surface of the substrate, which canaccumulate from performing operation 503. Exposing the substrate tohydrogen may form tin hydrates which are volatile at the chosensubstrate temperature, which may then be pumped from the processingchamber. The substrate may be exposed to the plasma treatment for aduration greater than 0 seconds and less than 5 seconds. The duration ofplasma exposure may depend on the amount of tin on the surface. Forexample, in some implementations, the amount of tin may be determined byevaluating tin lines in an emission spectrum. In some implementations,the plasma may be turned off when the tin lines in an emission spectrumdisappear. In some implementations, the substrate is exposed to theplasma for about 5 seconds. In some implementations, the substrate isexposed to the plasma for a duration greater than about 5 seconds. Invarious implementations, the plasma treatment may include introducing ahydrogen gas and igniting a plasma. Operation 505 may be performed inthe same chamber as in operation 501 and/or 503. Note that althoughoperation 505 may be performed by exposing the substrate to hydrogenplasma, in some implementations a different chemistry may be used toremove tin or tin oxide buildup on the surface of the material to beetched. For example, in some implementations, ammonia (NH₃) plasma maybe used.

In some implementations, operation 505 may be performed in a separatechamber. For example, in some implementations, the substrate may bemoved or shuttled to the first station/chamber where operation 501 wasperformed, or may be moved or shuttled to a third station/chamber toperform operation 505. Note that movement or shuttling of the substratebetween chambers may be performed without breaking vacuum.

In operation 507, it is determined whether the amount etched issufficient to achieve the desired amount to be etched. If the desiredremaining thickness has not yet been achieved, operations 501-505 may beoptionally repeated. Note that in some implementations, operation 505may only be performed every n cycles of performing operations 501 and503, where n is an integer greater than or equal to 1. Where n is 1,operation 505 is performed in every cycle. In various implementations,operation 505 is performed in every cycle. In another example, operation505 may be performed every 2 cycles of performing operations 501 and 503(where n is 2) such that the following operations may be performed toetch a substrate: (1) exposure to fluorine-containing plasma, (2)exposure to Sn(acac)₂ vapor, (3) exposure to fluorine-containing plasma,(4) exposure to Sn(acac)₂ vapor, (5) exposure to hydrogen plasma, and(6) repeat (1)-(5).

In accordance with some implementations of the disclosure, isotropicatomic layer etch utilizes a low-bias plasma during the modificationstep. When etching metal oxides such as Al₂O₃, this involves a zero-biasfluorine plasma to form aluminum-fluoride at the surface of the oxidefilm. This step is self-limiting to a few monolayers as the datadescribed below indicate.

During the following vapor removal step, Sn(acac)₂ reacts with thefluorinated top surface of the film via the ligand exchange mechanismand etches away the fluorinated layer. As the vapor treatment brings nodirectional energy such as ion energy from a plasma sheath to the wafer,the vapor step etches the metal fluoride isotropically. The overallsequence of reactions can be summarized in the following way: (1) Createa fluorinated shallow surface layer of ˜1.5 nm in a low bias fluorineplasma. The plasma may be based on CF₄ or NF₃, for example. (2) Withoutplasma, apply Sn(acac)₂ vapor while the substrate is heated to anelevated temperature (for example, 200 C) to perform a ligand exchangereaction between the fluorine and the acac ligands. (3) Pump awayvolatile reaction by-products. (4) Apply a brief hydrogen plasma flashto the surface of the substrate to remove non-volatile tin by-productsfrom the substrate surface. (5) Return to step (1) and repeat.

Anisotropy can be introduced to the etch process in a controlled fashionby turning on plasma bias during the plasma fluorination step in acontrolled manner. Data have been obtained showing that the depth offluorination can be controlled via bias energy. Fluorine ions willadvance deeper into the metal oxide film before they can be stopped iftheir incipient ion energy acquired during the acceleration in theplasma sheath is greater.

To better understand fluorination, Al₂O₃ films were exposed to variousfluorination conditions after which a set of characterization techniqueswere employed to understand changes to the film. Angle resolved x-rayphotoelectron spectroscopy (ARXPS) was used to measure fluorinationdepth and total material loss.

FIG. 6 illustrates a graph showing aluminum oxide thickness and aluminumoxy-fluoride thickness under various conditions, using ARXPScharacterization of the surface following single fluorination, inaccordance with implementations of the disclosure. As indicated,fluorination depth was shown to be dependent on plasma density and ionenergy. However, cathode bias (influencing ion energy) demonstrated thehighest impact on fluorination depth, to a significantly greater extentthan plasma density.

FIG. 7 illustrates surface fluorination depth using zero-bias plasma, inaccordance with implementations of the disclosure. The fluorinationdepth after plasma treatment was probed via depth-resolved XPS. Inagreement with the results shown at FIG. 6, the fluorination depth wasfound to be limited down to approximately 1.5 nm. More specifically, asample having a 100 angstrom surface thickness of Al₂O₃ over a silicondioxide (thermal oxide) layer (1000 angstrom thickness) over a siliconsubstrate was profiled. In the illustrated graph, measured atomicpercentages for the elements aluminum, oxygen, and fluorine as afunction of sputter time (seconds) are shown. Measurements are shownboth before and after the application of a fluorination plasma under azero bias condition.

Before the application of the fluorination plasma, the atomic percentageof oxygen is shown by curve 700; the atomic percentage of aluminum isshown by curve 702; and the atomic percentage of fluorine is shown bycurve 704. As indicated, the atomic percentage of aluminum drops off ataround 200 seconds, which corresponds to the complete sputtering of the100 angstrom (10 nm) thickness of the aluminum oxide layer. Thus,approximately 1 nm of thickness is sputtered every 20 seconds. Theatomic percentage of oxygen, shown by curve 700, increases after about200 seconds, as the sputter reaches the silicon dioxide layer. Theatomic percentage of fluorine, shown by curve 704, is zero throughout,as the fluorination plasma has yet to be applied.

After the application of the fluorination plasma, the atomic percentageof oxygen is shown by curve 706; the atomic percentage of aluminum isshown by curve 708; and the atomic percentage of fluorine is shown bycurve 710. As can be seen, the atomic percentage of fluorine drops tonear zero within about 30 seconds of sputter time, which corresponds toa depth of about 15 angstroms (1.5 nanometers). Thus, with zero bias,the fluorination plasma achieved a fluorine diffusion depth of about 15angstroms.

FIGS. 8A-E are STEM images of a cross section of a fluorinated film, inaccordance with implementations of the disclosure. FIG. 8A shows thepre-fluorination cross-section, including a surface layer of aluminumoxide having a thickness of about 11 nanometers (nm), over a layer ofthermal silicon dioxide. FIG. 8B shows the result of fluorinationwithout bias. FIG. 8D shows a close-up view of a portion of this result.

FIG. 8C shows the result of fluorination with a 100V bias. FIG. 8E showsa close-up view of a portion of this result.

As indicated by these images, the modified depth (indicated by darkergrey) was 5.7 nm without bias, and 6.7 nm with 100V bias. Thefluorinated depth (indicated by light grey top) was 1.4 nm without bias,and 2.5 nm with 100V bias. The ‘c’ lattice constant of Al₂O₃ is ˜1.3 nm.As can be seen from the images, the fluorinated depth has increased as aresult of the application of a 100V bias.

FIGS. 9A and 9B illustrate film loss following a 30 second fluorineplasma exposure versus a 300 second fluorine plasma exposure,respectively, in accordance with implementations of the disclosure. Asshown, the 300 second fluorine plasma exposure did not produceadditional film loss of significance beyond that of the 30 secondfluorine plasma exposure.

FIG. 10 is a graph showing Spectral Ellipsometry (SE) characterizationof film loss, demonstrating the self-limiting nature of the fluorinationand Sn(acac)₂ exposure, in accordance with implementations of thedisclosure.

As shown, the zero-bias fluorination process is self-limiting. The ˜5 Aloss is likely due to refractive index change during fluorination.

Additionally, the results show that the material removed in one ALEcycle may weakly depend on Sn(acac)₂ application time. However, there issome material left on the surface which limits the ability to etch tothe full extent of the fluorination depth (˜15 A).

As noted, the fluorinated depth increases with the application of abias. Accordingly, during the Sn(acac)₂ vapor step more metal fluoridewill be removed from the film having a bias applied during thefluorination step (e.g. 100V) than in the zero-bias case. As thefluorination depth only increases on surfaces parallel to the plasmasheath edge but not on those perpendicular to it, an anisotropy in thefollowing removal step can be achieved. That is, more material will beremoved from horizontal than from vertical surfaces.

FIG. 11 is a graph illustrating calculated fluorination depth as afunction of fluorine ion energy, in accordance with implementations ofthe disclosure. As shown, increased ion energy results in increasedfluorination depth. For example, to achieve greater than 1 nmfluorination depth would require ion energy greater than 100 eV. Thus,the amount of anisotropy of the ALE process increases with increasingion energy during the fluorination step.

As noted above, the ligand exchange operation may not fully consume thefluorinated portion of the substrate in a single operation, as it may beself-limited due to residue build-up, and a hydrogen plasma may beapplied to remove the residue. Therefore, in some implementations,ligand exchange and hydrogen plasma operations can be repeated multipletimes per each fluorination operation.

FIG. 12 illustrates a method for performing ALE using multiple ligandexchange and plasma cleaning operations per single surface modificationoperation, in accordance with implementations of the disclosure. Theillustrated method is described with reference to fluorine-containingplasma for surface modification, Sn(acac)₂ for ligand exchange, andhydrogen plasma for residue removal. However, in variousimplementations, the method can be applied for any other set of specificchemistries for surface modification, ligand exchange, and residueremoval.

At method operation 1201, using a fluorine plasma, a fluorinated surfacelayer is created, having a depth that is controlled by the magnitude ofthe bias voltage during the fluorine plasma exposure. In someimplementations, the plasma may be generated using CF₄ or NF₃. It willbe appreciated that the application of a bias voltage will not onlyincrease the depth of fluorination, but also the anisotropy of theoverall etch process, as the fluorination depth is increased through theapplication of the bias in a directional manner (normal to thesubstrate/wafer plane).

At method operation 1203, without plasma, Sn(acac)₂ vapor is appliedwhile the substrate is heated to an elevated temperature (for example,200 C) to perform a ligand exchange reaction between the fluorine andthe acac ligands. It is noted that a single Sn(acac)₂ vapor applicationmay only perform ligand exchange with the top layers of the fluorinatedfilm, and therefore may not completely consume the entire fluorinatedfilm. This may especially be true in the case where the fluorinateddepth has been increased through the application of a bias voltage (ascompared to a zero-bias fluorination plasma). Following the Sn(acac)₂vapor application, volatile reaction by-products are pumped away.However, as noted, there may be a build-up of (non-volatile)tin-containing residues that remain on the surface, and which preventfurther ligand-exchange reaction (and subsequent removal of material)from occurring during the Sn(acac)₂ vapor application.

Therefore, at method operation 1205, a brief hydrogen plasma flash isapplied to the surface of the substrate to remove non-volatile tinby-products from the substrate surface.

As noted, the vapor application occurring at method operation 1203 maynot have consumed the entire fluorinated surface layer. Hence, at methodoperation 1207, it is determined whether the fluorinated layer has beenconsumed by the ligand exchange. If not, then the method returns tooperation 1203, to repeat the Sn(acac)2 and hydrogen plasma exposuresuntil the fluorinated layer has been consumed. It will be appreciatedthat the number of cycles of method operations 1203 and 1205 required toentirely consume the fluorinated layer may be experimentallypredetermined. Consequently, determining whether the fluorinated layerhas been consumed at method operation 1207 may be defined by determiningwhether the predetermined number of cycles has been performed.

When the entire fluorinated layer has been consumed, or if apredetermined number of cycles necessary to fully consume thefluorinated layer has been performed, then at operation 1209, it isdetermined whether the film has been etched to the desired thickness. Ifnot, then the method returns to operation 1201 to perform the surfacefluorination.

The process (including method operation 1201, 1203, 1205, and 1207) isrepeated until the film has been etched to the desired thickness, oruntil a predetermined number of cycles has been completed so as toachieve the desired thickness.

It will be appreciated that the foregoing process is faster thanconventional ALE processes due to the performance of a singlefluorination operation for multiple cycles of the Sn(acac)₂ exposure andhydrogen plasma exposure, as opposed to performing the fluorinationoperation with each instance of the Sn(acac)₂ and hydrogen plasmaexposures. This can increase throughput for the ALE process.Furthermore, by reducing the number of fluorination operations, it ispossible to preserve selectivity to a mask (e.g. silicon oxide mask)that may be present on the substrate surface, and which may besusceptible to degradation through multiple fluorination operations. Bycontrast, the ligand exchange is selective and does not affect the mask.

FIG. 13A conceptually illustrates a cross section of a substrate surfacefeature 1300, and performance of an anisotropic ALE process performedthereon, in accordance with implementations of the disclosure. The topsurface of the substrate may include a mask 1302 to prevent otherportions of the substrate from being etched. In the illustratedimplementation, an isotropic ALE process is performed by performing ALEusing a surface modification mechanism with zero bias. By performing thesurface modification with zero bias, then the effect of the surfacemodification will be isotropic, producing conversion of availablesurface species to approximately uniform depth in an omnidirectionalmanner. As noted above, the surface modification may be diffusionlimited to produce the depth of surface modification.

Because the surface modification has been isotropically performed, thenthat portion which has been modified is available for removal by thesubsequent removal operation (e.g. via a ligand exchange mechanism). Theresult is an ALE process that is isotropic due to the lack of bias beingapplied during the surface modification operation. As noted above, insome implementations, a single surface modification may penetrate to adepth that is greater than that which can be removed by a single removaloperation; and thus in some implementations, multiple removal and plasmaclean operations are performed in succession in order to fully removethe entire portion that has been modified by the single surfacemodification operation.

The initial surfaces 1304 of the feature 1300 are shown, and successivecycles of the zero bias etch process isotropically deepen the surfaces1304 of the feature 1300. The resulting surfaces of the feature 1300following successive etch cycles are respectively shown by the surfaces1306, 1308, 1310, and 1312. For example, following one etch cycle, thefeature 1300 is isotropically etched so as to have surfaces 1306;following a second etch cycle, the feature 1300 is isotropically etchedso as to have surfaces 1308; etc.

FIG. 13B conceptually illustrates a cross section of a substrate surfacefeature, and performance of an isotropic ALE process performed thereon,in accordance with implementations of the disclosure. The implementationof FIG. 13B is similar to that of FIG. 13A, except that during thesurface modification operation, a bias voltage is applied, whichintroduces a degree of directionality to the surface modificationoperation. With increased bias power, the ions will be driven deeper inthe downward vertical direction (orthogonal to the plane of thesubstrate surface) than the horizontal direction. The result is thatsurface modification will occur to greater depths in the downwardvertical direction, and to reduced depths in the horizontal direction,as compared to a zero-bias surface modification. Then as the amount ofmaterial that has been converted by the surface modification operationdetermines that which is available for removal by the removal operation,then the result will be an anisotropic ALE exhibiting greater etch ratealong the downward vertical direction and reduced etch rate along thehorizontal direction, as compared to the zero-bias case.

With continued reference to FIG. 13B, the initial surfaces 1304 of thefeature 1300 are again shown. However, in contrast to the zero biasetch, successive cycles of the biased etch process anisotropicallydeepen the surfaces 1304 of the feature 1300. The resulting surfaces ofthe feature 1300 following successive anisotropic etch cycles arerespectively shown by the surfaces 1314, 1316, 1318, and 1320. Forexample, following one etch cycle, the feature 1300 is anisotropicallyetched so as to have surfaces 1314; following a second etch cycle, thefeature 1300 is anisotropically etched so as to have surfaces 1316; etc.Additionally, in some implementations, the vertical surfaces can becoated with a protective polymer such that they are excluded from theALE cycle all together. In this manner, one can avoid etching thevertical surfaces completely.

In sum, during the modification step in an ALE process, the depth of themodified layer (along the direction of the bias flux, or generallyorthogonal to the substrate plane) can be controlled via the biasvoltage applied during that step. The depth modification is generallylimited to those surfaces parallel to the plasma sheath edge. Because ofthis, anisotropy can be introduced in a controlled way by controllingthe amount of bias applied during plasma fluorination. That is, therelative etch rates in the vertical direction (orthogonal to thesubstrate plane) versus the horizontal direction (parallel to thesubstrate plane) can be tuned, with the ratio of vertical to horizontaletch increasing with increased bias power.

It will be appreciated that the bias power can be tuned for particularapplications and ALE chemistries, and that there may be trade-offsrelated to the bias power. For example, as bias power is increased (e.g.by controlling bias voltage), ion implantation may occur to greaterdepths, producing greater surface modification depth, and increasedanisotropy. However, as bias power is increased, more energy is impartedto the ions, which may also produce film loss due to reactive-ionetching and/or sputtering. Thus, in some implementations, bias power istuned to provide a desired depth of surface modification, while alsosubstantially avoiding film loss or tolerating an acceptable level offilm loss for the given ALE application. One can control the bias to alevel below the sputter threshold of the material that is being etched.That way, premature film loss can be minimized.

In some implementations, the degree of anisotropy can be increased via apolymerizing plasma step prior to fluorination to deposit a polymerliner inside the structure that is to be etched. This liner can beopened before or during the first part of the fluorination step on thebottom surface only (e.g. by photoresist, photolithography, and ionetch) but would remain intact for sidewalls, thereby protecting them.

FIGS. 14A-D illustrate a process for providing increased anisotropythrough deposition of a passivation layer, in accordance withimplementations of the disclosure. FIG. 14A conceptually illustrates across section of a substrate surface feature 1400. A passivation layer1402 is deposited in the feature, as shown at FIG. 14B. The passivationlayer is a protective liner that protects the underlying surface frombeing etched during a subsequent ALE process. In variousimplementations, the passivation layer can consist of a polymermaterial, and inorganic material, or any other material capable ofprotecting feature surfaces from being etched during a subsequent ALEprocess. Furthermore, the passivation layer can be deposited by anysuitable technique, including without limitation, CVD, ALD, etc.

At FIG. 14C, the bottom of the passivation layer 1402 is opened,exposing the underlying substrate material for etching. At FIG. 14D, ananisotropic ALE process is carried out (using a bias voltage during thesurface modification step). As shown, the bottom of the feature isetched, while the feature's sidewalls 1401 are protected by thepassivation layer. In some implementations, the passivation layer 1402is not etched by the ALE process. Whereas in other implementations, thepassivation layer 1402 is configured to be partially or fully etched bythe ALE process. In such implementations, the passivation layer 1402acts to prevent or delay the onset of etching of the feature's sidewalls1401, providing for increased anisotropy of the overall process.

Various implementations described herein may be performed in a plasmaetch chamber such as the Kiyo, available from Lam Research Corporationin Fremont, Calif. In various implementations, a substrate may beshuttled between an etching chamber and a vapor chamber without breakingvacuum.

Disclosed implementations may be performed in any suitable chamber orapparatus, such as the Kiyo® or Flex, both available from Lam ResearchCorporation of Fremont, Calif. In some implementations, disclosedimplementations may be performed in a cluster tool, which may containone or more stations. FIG. 15 illustrates a cluster tool 1500, inaccordance with implementations of the disclosure. In variousimplementations, one station 1501 may include a module for etching whileanother station 1503 includes a module for exposing to vapor (e.g., avapor chamber). In some implementations, a third station 1505 includes amodule for exposing to a plasma.

In some implementations, an inductively coupled plasma (ICP) reactor maybe used. Such ICP reactors have also been described in U.S. PatentApplication Publication No. 2014/0170853, filed Dec. 10, 2013, andtitled “IMAGE REVERSAL WITH AHM GAP FILL FOR MULTIPLE PATTERNING,”hereby incorporated by reference for the purpose of describing asuitable ICP reactor for implementation of the techniques describedherein. Although ICP reactors are described herein, in someimplementations, it should be understood that capacitively coupledplasma reactors may also be used. With reference to FIG. 16, an exampleetching chamber or apparatus may include a chamber 1601 having ashowerhead or nozzle 1603 for distributing fluorine-containing gases(1605), hydrogen gas (1607), or Sn(acac)₂ vapor (1609) or otherchemistries to the chamber 1601, chamber walls 1611, a chuck 1613 forholding a substrate or wafer 1615 to be processed which may includeelectrostatic electrodes for chucking and dechucking a wafer. The chuck1613 is heated for thermal control, enabling heating of the substrate1615. The chuck 1613 may be electrically charged using an RF powersupply 1617 to provide a bias voltage in accordance with implementationsof the disclosure (e.g. at a voltage in the range of approximately 20 to200V, 13.56 Mhz). An RF power supply 1619 is configured to supply power(e.g. in the range of approximately 100 W to 3 kW, at 13.56 Mhz) to acoil 1621 to generate a plasma, and gas flow inlets for inletting gasesas described herein. Though an ICP chamber is shown, in otherimplementations, a CCP chamber can be utilized. In variousimplementations, the chamber walls 1611 may be fluorine-resistant. Forexample, the chamber walls 1611 may be coated with silicon-containingmaterial (such as silicon or silicon oxide) or carbon-containingmaterial (such as diamond) or combinations thereof such thatfluorine-containing gases and/or plasma may not etch the chamber walls1611. Modification chemistry gases for chemisorption (such asfluorine-containing gases for generating fluorine-containing plasma)and/or vapor exposure (such as Sn(acac)₂) may be flowed to the chamber1601. In some implementations, a hydrogen gas 1607 may be flowed to thechamber to generate a hydrogen plasma for removing tin, tin fluoride ortin oxide residues. In some implementations, the chamber walls areheated to support wall cleaning efficiency with a hydrogen plasma. Insome implementations, an apparatus may include more than one chamber,each of which may be used to etch, deposit, or process substrates. Thechamber or apparatus may include a system controller 1623 forcontrolling some or all of the operations of the chamber or apparatussuch as modulating the chamber pressure, inert gas flow, plasma power,plasma frequency, reactive gas flow (e.g., fluorine-containing gas,Sn(acac)₂ vapor, etc.); bias power, temperature, vacuum settings; andother process conditions.

FIG. 17 shows a control module 1700 for controlling the systemsdescribed above, in accordance with implementations of the disclosure.For instance, the control module 1700 may include a processor, memoryand one or more interfaces. The control module 1700 may be employed tocontrol devices in the system based in part on sensed values. Forexample only, the control module 1700 may control one or more of valves1702, filter heaters 1704, pumps 1706, and other devices 1708 based onthe sensed values and other control parameters. The control module 1700receives the sensed values from, for example only, pressure manometers1710, flow meters 1712, temperature sensors 1714, and/or other sensors1716. The control module 1700 may also be employed to control processconditions during reactant delivery and plasma processing. The controlmodule 1700 will typically include one or more memory devices and one ormore processors.

The control module 1700 may control activities of the reactant deliverysystem and plasma processing apparatus. The control module 1700 executescomputer programs including sets of instructions for controlling processtiming, delivery system temperature, pressure differentials across thefilters, valve positions, mixture of gases, chamber pressure, chambertemperature, wafer temperature, RF power levels, wafer ESC or pedestalposition, and other parameters of a particular process. The controlmodule 1700 may also monitor the pressure differential and automaticallyswitch vapor reactant delivery from one or more paths to one or moreother paths. Other computer programs stored on memory devices associatedwith the control module 1700 may be employed in some implementations.

Typically there will be a user interface associated with the controlmodule 1700. The user interface may include a display 1718 (e.g. adisplay screen and/or graphical software displays of the apparatusand/or process conditions), and user input devices 1720 such as pointingdevices, keyboards, touch screens, microphones, etc.

Computer programs for controlling delivery of reactant, plasmaprocessing and other processes in a process sequence can be written inany conventional computer readable programming language: for example,assembly language, C, C++, Pascal, Fortran or others. Compiled objectcode or script is executed by the processor to perform the tasksidentified in the program.

The control module parameters relate to process conditions such as, forexample, filter pressure differentials, process gas composition and flowrates, temperature, pressure, plasma conditions such as RF power levelsand the low frequency RF frequency, cooling gas pressure, and chamberwall temperature.

The system software may be designed or configured in many differentways. For example, various chamber component subroutines or controlobjects may be written to control operation of the chamber componentsnecessary to carry out the inventive deposition processes. Examples ofprograms or sections of programs for this purpose include substratepositioning code, process gas control code, pressure control code,heater control code, and plasma control code.

Although the foregoing implementations have been described in somedetail for purposes of clarity of understanding, it will be apparentthat certain changes and modifications may be practiced within the scopeof the disclosed implementations. It should be noted that there are manyalternative ways of implementing the processes, systems, and apparatusof the present implementations. Accordingly, the present implementationsare to be considered as illustrative and not restrictive, and theimplementations are not to be limited to the details given herein.

What is claimed is:
 1. A method for performing atomic layer etching (ALE) on a substrate, comprising: performing a surface modification operation on a substrate surface, the surface modification operation configured to convert at least one monolayer of the substrate surface to a modified layer, wherein a bias voltage is applied during the surface modification operation, the bias voltage configured to control a depth of the substrate surface that is converted by the surface modification operation; performing a removal operation on the substrate surface, the removal operation configured to remove at least a portion of the modified layer from the substrate surface, wherein removing the portion of the modified layer includes applying thermal energy to effect desorption of the portion of the modified layer.
 2. The method of claim 1, wherein removing the portion of the modified layer includes a ligand exchange reaction.
 3. The method of claim 2, wherein the thermal energy is applied simultaneously with the ligand exchange reaction.
 4. The method of claim 2, wherein the thermal energy is applied after the ligand exchange reaction.
 5. The method of claim 1, wherein the surface modification operation is configured to diffuse ions into the substrate surface to the depth as controlled by the bias voltage.
 6. The method of claim 1, wherein the bias voltage is applied during part of the surface modification operation, the part during which the bias voltage is applied to increase an amount of the depth in a vertical direction that increases anisotropy of the ALE, and a portion during which the bias voltage is not applied to increase the depth in a non-vertical direction that increases isotropy of the ALE.
 7. The method of claim 1, wherein the bias voltage does not exceed approximately 100 V.
 8. The method of claim 1, wherein performing the surface modification operation includes exposing the substrate surface to a plasma, wherein the exposure to the plasma is configured to convert the at least one monolayer of the substrate surface to the modified layer.
 9. The method of claim 8, wherein exposing the substrate surface to the plasma includes using a remote plasma source to generate the plasma, and flowing the plasma from the remote plasma source into a chamber in which the substrate is disposed.
 10. The method of claim 8, wherein exposing the substrate surface to the plasma includes inductively coupling power into a chamber in which the substrate is disposed.
 11. The method of claim 8, wherein exposing the substrate surface to the plasma includes capacitively coupling power into a chamber in which the substrate is disposed.
 12. The method of claim 1, wherein the substrate surface includes a metal, metal oxide, metal nitride, metal phosphide, metal sulfide, metal arsenide, or metal compound.
 13. A method for performing atomic layer etching (ALE) on a substrate, comprising: performing a surface modification operation on a substrate surface, the surface modification operation including exposing the substrate surface to a first plasma that converts at least one monolayer of the substrate surface to a modified layer, wherein a bias voltage is applied during the surface modification operation, the bias voltage being configured to control a depth of the substrate surface that is converted by the surface modification operation, wherein the bias voltage is configured to accelerate ions from the first plasma towards the substrate surface without substantially etching the substrate surface; performing a removal operation on the substrate surface, the removal operation including removing at least a portion of the modified layer from the substrate surface, wherein removing the portion of the modified layer includes applying thermal energy to effect desorption of the portion of the modified layer; performing a clean operation on the substrate surface, the clean operation including removing residues generated by the removal operation from the substrate surface, the clean operation further including exposing the substrate surface to a second plasma, wherein the residues are volatilized by the exposure to the second plasma.
 14. The method of claim 13, wherein removing the portion of the modified layer includes a ligand exchange reaction.
 15. The method of claim 14, wherein the thermal energy is applied simultaneously with the ligand exchange reaction.
 16. The method of claim 14, wherein the thermal energy is applied after the ligand exchange reaction.
 17. The method of claim 13, wherein the substrate surface includes a metal, metal oxide, metal nitride, metal phosphide, metal sulfide, metal arsenide, or metal compound.
 18. A method for performing atomic layer etching (ALE) on a substrate, comprising: performing a surface modification operation on a substrate surface, the surface modification operation including exposing the substrate surface to a halogen-containing plasma that converts at least one monolayer of the substrate surface to a modified layer, wherein a bias voltage is applied during the surface modification operation, the bias voltage being configured to control a depth of the substrate surface that is converted by the surface modification operation, wherein the bias voltage is configured to accelerate ions from the first plasma towards the substrate surface without substantially etching the substrate surface; performing a removal operation on the substrate surface, the removal operation including removing at least a portion of the modified layer from the substrate surface, wherein removing the portion of the modified layer includes a ligand exchange reaction and applying thermal energy to effect desorption of the portion of the modified layer;
 19. The method of claim 18, wherein the substrate surface includes a metal, metal oxide, metal nitride, metal phosphide, metal sulfide, metal arsenide, or metal compound; wherein the exposure to the halogen-containing plasma forms a metal halide.
 20. The method of claim 18, wherein exposing the substrate surface to the halogen-containing plasma includes introducing a halogen-containing gas into a chamber in which the substrate is disposed, and igniting the halogen-containing plasma.
 21. The method of claim 20, wherein igniting the halogen-containing plasma includes inductively coupling power into the halogen-containing gas.
 22. The method of claim 20, wherein igniting the halogen-containing plasma includes capacitively coupling power into the halogen-containing gas.
 23. The method of claim 8, wherein exposing the substrate surface to the halogen-containing plasma includes using a remote plasma source to generate the halogen-containing plasma, and flowing the halogen-containing plasma from the remote plasma source into a chamber in which the substrate is disposed. 